Caption: Deforestation. Example of deforestation in the Langtang valley, Nepal, near the border with the Chinese region of Tibet. The need for firewood and more space for crops has led the villagers to remove the trees and to develop terraced agriculture. On this scale deforestation is a cause of landslip and soil erosion. On a global scale the exploitation of forest regions, together with the increase of the pollution caused by human activities, is considered a key factor in the global warming effect.